The present invention relates to color printing generally and, more particularly, to adaptive error diffusion circuits and methods in a color image processor.
Generally, when a color image is reproduced in an image processor such as a color printer, brightness and color distortion occur according to the color image processing type and the characteristics of the color printer to which the adopted type of image processing is applied. Since human eyes are very sensitive to the brightness of an image and the variation of color, distortion of brightness and color must be appropriately compensated to obtain good color printing and reproduction quality.
An error diffusion process is a known method for processing brightness and color. This process is widely used in color image printers having limited primary colors of red, green, blue, cyan, magenta and yellow, since their image reproducing capability is excellent. When a continuous gray scale image is binarized, an error is generated. The error diffusion process is a technique used to distribute the error to neighboring pixels. In consideration of this error, binarization is executed when the next pixel is processed. Consequently, spatial resolution is improved as compared with an ordered dithering process. Examples of error diffusion processes are described in further detail in an article written by Robert Ulichney entitled Digital Halftoning (see Chapter 8, Dithering with blue noise) published by MIT press, as well as in the Method And System For Reproducing Monochromatic And Color Images Using Ordered Dither And Error Diffusion of U.S. Pat. No. 5,031,050 and Method And System For Printing In One Or More Color Planes With Improved Control Of Eorror Diffusion, U.S. Pat. No. 5,140,432 issued to Chan.
In typical conventional error diffusion circuits, a video signal applied to the input terminal line is added to the output of an error diffusion filter to obtain a first value. The first value generated is applied to a threshold circuit. The output of the threshold circuit and the first value are added to generate a second value. The second value generated is the error value, this error value is applied to the error diffusion filter.
A conventional error diffusion circuit exhibits good intermediate tones in a monochromatic image. I have noticed however, that it is difficult to reproduce an original color image in an ink jet or electrophotographic type color printer because the characteristics of the printer are not considered by conventional error diffusion circuits. Consequently, I have found that with conventional circuits, the color image may be dark and discolored as a result of ink bleed and color overlap between a current pixel and one of its neighboring pixels.
Various attempts have been made to improve the error diffusion process and the overall quality of printer image reproduction. It has been my observation however, that previous attempts do not address absorption error with respect to the color of the neighboring pixels and the current pixel being printed, nor do they take into account the characteristics of different printers in calculating the absorption error.
Other examples include the Method And System for Enhancing The Quality Of Both Color And Black And White Images Produced By Ink Jet And Electrophotographic Printers of Chan et al. (U.S. Pat. No. 5,111,302) has a system for reducing undesirable color contrast or "graininess" in ink jet and electrophotographic printers by assigning differing loading variations, addressed from a grey table, to a pixel to be printed according to pixel values of an image.
The Method And Apparatus For Imbedding Controlled Structure For Gray Scale Rendering of Tai (U.S. Pat. No. 5,260,807) discloses an apparatus for providing stable dots and spreading out information over a cell for better resolution and reproduction of gray scales. The apparatus uses a full dot process to form dots to their maximum allowable dot size at high priority pixels before beginning formation of dots for the next highest priority pixels, a partial dot process for providing a dot of the same size for each pixel in a cell before building a dot at any particular pixel to the next larger size, or a mixed dot process which combines the full and partial dot processes.
The High Speed Digital Error Diffusion Process For Continuous Tone Image-To-Binary Image Conversion of Ghaderi (U.S. Pat. No. 5,172,247) discloses an error diffusion process which utilizes a look-up table to reduce the amount of time required to perform calculations for the error diffusion process.
Each of these references endeavors to improve the error diffusion process. I have noticed however, that none of these endeavors adequately addresses the problem of absorption error with respect to the color of the current pixel being printed relative to its neighboring pixels, nor do they take into account the characteristics of different printers in calculating absorption error.
It also seems to me that in conventional ink jet printers, every pixel is printed in a larger size than is ideal, and that this causes overlap between a current pixel and one or more of its neighboring pixels. Therefore, when a picture image is printed, the brightness becomes darker and color variation partially occurs. Ideally, the absorption of a printed pixel is 255, while the absorption of a pixel that is not printed is zero. When brightness and color vary however, the absorption of a printed pixel is between 50-1200 (with the actual value differing according to the color of the current pixel and its neighboring pixel), and the absorption of a pixel that is not printed is zero. It is my opinion that since the errors are very large in comparison with an ideally printed picture image and the errors differ according to the color of the neighboring pixel, these errors should be considered when a pixel is processed. I have also observed that the calculated result may vary with the printer type and resolution; nevertheless, the aforementioned disadvantages occur in conventional ink jet printers because pixels are processed on the basis of an ideal picture image without consideration of diversity between printer characteristics.